The basic problem of the invention is to provide an arrangement which modifies a high frequency magnetic field, is effective from an energy point of view and sets in a well defined manner to configure the areas which are to be "shielded". Furthermore, the arrangement should be easy to manufacture and operate.
In high frequency induction welding the need for directing a magnetic field from the "welding inductor" towards the welding areas and to shield other areas from magnetic fields that produce an electric current has been recognized.
In a known method a short circuited loop of electrically conducting material, for instance copper, is arranged around the area which is to be shielded from the magnetic field. The field/the magnetic flow induces current in the loop and this current produces a counter-acting magnetic flow to reduce the net flow in the "shielded" area. This method, however, produces a heavy heat dissipation in the loop as a result of the induced current. The heat dissipation is such that considerable cooling is necessary when operating the welding tool at a production rate. Such a cooling imposes losses which cause deterioration in efficiency.
Another method employs the placement of ferro magnetic material having a high permeability, for instance soft iron, adjacent to the welding area (areas) for "collecting", in a known manner, the magnetic flow in the high permeability material. This principle is useful in low frequency applications, for instance power transformers, however in high frequency applications its usefillness is limited. The term "high frequency" in this particular application is intended to mean a frequency of an order of at least kHz, usually 10-50 kHz. Higher frequencies may also be used, especially when only small penetration depths are needed in the electrically conducting material in the welding area. A "concentration" of energy to small penetration depths, however, may result in burning through of the material. At very high frequencies the dielectric phenomena will also contribute to the end result.